The objectives of this project are to develop a vaccine against cutaneous leishmaniasis by both defining protective leishmanial antigens and investigating the mechanisms responsible for protective immunity using experimental infections of BALB/c mice with Leishmania major. One antigen fraction (9) induces protection, is recognized by a T cell line that adoptively transfers protection and belongs to the TH1 subset of CD4+ cells. In contrast, another antigen fraction (1) was not protective and stimulated the TH2 CD4+ subset. We have established additional T cell lines that recognize these two antigen fractions, and have confirmed the strong association of fraction 9 with protection and TH1 T cells, and fraction 1 with exacerbation and TH2 T cells. In addition, we analyzed the lymphokines produced by T cells from infected mice that had been immunized with the different fractions. T cells from animals protectively immunized with fraction 9 produce large amounts of IFN-gamma, while T cells from fraction 1 immunized mice produce elevated levels of IL-4. Taken together, these results suggest that certain parasite antigens induce protection by stimulation of TH1 cells, while other parasite antigens counteract protective T cells by stimulation of the TH2 subset of CD4+ T cells. Furthermore, we found that the mechanism by which TH2 cells exacerbate infection may be by modulation of IFN-gamma induced activation. Thus, macrophages exposed to IL-4, GM-CSF, and/or IL-3 exhibit decreased parasite killing when activated with IFN-gamma. Three T cell clones, all of the TH1 phenotype, have been established to identify protective antigens. One clone, 9.1-2, recognizes a 10,000 m.w. protein and is protective. In addition to recognizing a L. major antigen, this clone recognizes antigens from L. mexicana, L. braziliensis L. panamensis, and L. donovani, but not Trypanosoma cruzi or Crithidia fasiculata. Finally, we have found that the protein recognized by this clone is released by promastigotes during in vitro culture.